The invention relates to a method and to an arrangement for the determination of the mass flow and the gas/liquid ratio in a multiphase mixture during conveying and to a conveying system for multiphase mixtures to carry out a method of this kind or including an arrangement of this kind.
The problem occurs in the conveying of multiphase mixtures such as crude oil, which also contains natural gas and frequently also water in addition to crude oil, that not only the total mass flow has to be determined, but also the gas/liquid ratio in the conveyed multiphase mixture. The knowledge of the mass flow and of the gas/liquid ratio is of great practical significance, for example for invoicing, for the control of the multiphase pump and for the process control, in particular for the setting of the conveying rate and for the quality monitoring.
An apparatus for the mixing and measurement of a multiphase fluid is known from WO 90/13859 which has a vessel, to which the gaseous and liquid portions are supplied, and a perforated header to drain the gaseous and liquid portions out of the container and to mix them. There are provided downstream of the header: a venturi nozzle to determine the mass flow and a densitometer to measure the density of the multiphase fluid. The apparatus known from WO 90/13859 has the disadvantage that a densitometer is used which works e.g. with gamma rays and requires high costs and a corresponding service effort.
In the publication WO 95/26494, an apparatus and a method are disclosed for the mixing, measurement and conveying of a multiphase mixture which use a buffer tank for the separation of the liquid and gaseous portions. The mass flow of the gaseous portion is detected separately in this method before the liquid and gaseous portions are combined together in a header again and conveyed on. Furthermore, the total mass flow can be calculated from the pressure of the gaseous portion, the pressure in the header and the level of the liquid in the buffer tank and from a venturi measurement.
The methods and apparatuses known from the prior art for the determination of the mass flow and of the gas/liquid ratio in a multiphase mixture have the disadvantage that they are complex and costly and, depending on the design, require a relatively large separator container and/or a cost-intensive and service-intensive densitometer. Moreover, the accuracy of the conventional methods is limited.